1. Field of the Invention
The present invention relates to improving the operation of electrodeless lamps.
2. Description of Related Prior Art
Under certain circumstances, electrodeless lamps will produce a discharge which is present only in a portion or portions of the bulb interior rather than being distributed substantially throughout the bulb volume.
One example of such a situation is where an electrodeless lamp having a fill wherein sulfur or selenium or compounds of these elements is the primary radiating component, utilizes a relatively small bulb.
Electrodeless lamps which utilize sulfur and selenium based fills are disclosed in co-pending application Ser. No. 071,027, filed Jun. 30, 1993, now U.S. Pat. No. 5,404,076 and application Ser. No. 604,487, filed Oct. 25, 1990, which are incorporated herein by reference. Such a lamp, at least in certain implementations relies on molecular radiation as the dominant source and is able to provide radiation principally within the visible range. Additionally, it is sometimes desirable to use small bulbs in such lamps, for example of less than 1/2" diameter in cases where a bright light source is needed. One such application is in LCD type projection television systems, where the necessity that the source approximate a so-called point light source dictates that the source be no more than about 1/4" inside diameter (ID).
U.S. Pat. No. 4,887,192, assigned in common with the present application, discloses microwave cavities for coupling to small bulbs, i.e., less than 1/2 inch, and is incorporated by reference herein.
However, it was found that when a bulb having a sulfur fill (including argon as a starting gas) of about 1/4" inch ID is installed in a cavity as disclosed in U.S. Pat. No. 4,887,192, successful operation is not achieved. Rather, a discharge which is isolated and floats near the top of the bulb is observed, and the spectrum which is outputted by the floating discharge is different than what is expected.
When an electrodeless lamp is excited with a coaxial termination fixture, an isolated discharge region may occur in the bulb at a position near the center conductor of the coaxial fixture, where the field is strongest. This leads to a phenomenon known as "arc attachment" wherein rupture of the bulb occurs at a position near the center conductor of the coaxial fixture.
A further problem in the operation of electrodeless lamps, and particularly those which emit visible light is that a "wobble" or "flicker" effect may be present. As used herein, the term "wobble" means any movement in the projected light which is observed by the viewer. This movement may be distracting to the viewer of the projected image, or even cause eye fatigue.
A reason for the wobble effect is that the bulb or envelope is itself visually imperfect and is rotated during operation. Thus, the envelope may have a perturbation, for example, an eccentricity, which would mean that the bulb is slightly out of round if, in the particular embodiment, it is a spherical envelope. The envelope in electrodeless lamps may be rotated while cooling fluid (e.g., compressed air) is directed at it to improve cooling, which allows the lamp to be operated at increased power densities, for example, see U.S. Pat. No. 4,485,332. It is the rotation of an envelope having a perturbation which results in the visual wobble or flicker when the bulb is rotated at the usual speed for cooling purposes of about 300 RPM.
U.S. Pat. No. 4,954,756 teaches to rotate bulbs at speed sufficient to equalize the thin boundary layer near the bulb wall by the application of centrifugal forces on the fill. For example at column 4, line 30 of that patent, it is stated that for bulbs of from 0.75 to 1.5 inch diameters rotation rates in the range of from 1500 to 2500 RPM may be used. It should be noted that between the low speed state and the high speed rotated state there is not a significant change in the spectral energy distribution of the bulb, although there is a change in the directional power distribution. The boundary layer effect described in U.S. Pat. No. 4,954,756 is different than the effect which is described herein, as in the boundary layer effect the discharge is distributed substantially throughout the interior of the envelope, but only is not present at the boundary layer, (which typically comprises about 5 to 20% of the total bulb volume). Such a boundary layer is typical in electrodeless lamps of the type described herein, and will exist even in lamps operated in accordance with the teachings of the present invention, wherein the discharge substantially fills the interior of the envelope. Moreover, since the redistribution effect in U.S. Pat. No. 4,954,756 is related to the centrifugal forces on the bulb, a rotation speed of at least 4500 RPM would be dictated for one-quarter inch ID bulbs in order to see this effect.